Understanding JavaScript's parseInt Function

The JavaScript parseInt() function is a built-in global function that is used to convert a string to an integer. Let's analyze the question to understand how this function works.

In the question, let stringToNum = parseInt("123ab"); is a piece of JavaScript code where the parseInt function is taking the string "123ab" as a parameter. It's important to remember that the parseInt function only converts the first number it finds in a string and continues the conversion until a character that isn't a number is found.

Therefore, when parsing the given string "123ab", it starts at the first character '1', which is a number. It continues to the next characters '2' and '3', which are also numbers. The function will stop at 'a' because it's not a number. Hence, the given string "123ab" is parsed into 123. For this reason, the correct answer is '123'.

An interesting note would be that if our code was let stringToNum = parseInt("ab123");, in this case, stringToNum would become NaN. This is because parseInt starts from the beginning of the string and stops when it encounters a non-numeric character. As 'a' and 'b' are not numbers, our parser immediately stops and hence 'NaN' is returned, which means 'Not a Number'.

Just remember, parseInt is a powerful tool that can convert strings that start with numbers to integers, but if the string starts with a non-numeric character, the function will return NaN.

As a best practice, it's recommended to always use the radix parameter of parseInt, which represents the numeral system to be used, to ensure predictable results. For example, if you want to use the decimal numeral system, you can specify the radix as 10: parseInt("123ab", 10);. This will also return '123'.